Linear alpha-olefins are an important material used in a comonomer, a cleaner, a lubricant, a plasticizer, etc. and is widely and commercially used. Particularly, 1-hexene and 1-octene are widely used as the comonomer for controlling the density of a polyethylene during preparing a linear low-density polyethylene (LLDPE).
In a preparation process of a common LLDPE, copolymerization of alpha-olefins, for example, 1-hexene, 1-octene, etc. with ethylene is performed to control density by forming branches at a polymer backbone.
Accordingly, in the preparation of an LLDPE having high comonomer content, the comonomer is a costly part. In order to resolve the drawback, various methods have been conducted.
In addition, the application field or the market size of alpha-olefins is dependent on the kind thereof, and technique on the selective production of a specific olefin is commercially very important. Recently, researches on a technique using a chromium catalyst for preparing 1-hexene or 1-octene with high selectivity via selective ethylene oligomerization are being actively conducted.
The conventional methods of commercially preparing 1-hexene or 1-octene include a shell higher olefin process (SHOP) of Shell Chemicals, a Ziegler process of Chevron Philips chemical, etc. Through the methods, alpha-olefins having wide distribution of C4-C20 may be obtained.
As a catalyst for trimerizing ethylene, a chromium-based catalyst using a ligand having the formula of (R1)(R2)X—Y—X(R3)(R4) is suggested. In the formula, X is phosphorus, arsenic or antimony, Y is a linking group such as —N(R5)-, and at least one of R1, R2, R3 and R4 have a polar or electron donating substituent.
In addition, a compound of (o-ethylphenyl)2PN(Me)P(o-ethylphenyl)2 including no polar substituent for at least one of R1, R2, R3 and R4 has been studied as a ligand not exhibiting catalyst activity with respect to 1-hexene under catalytic conditions (Chem. Commun., 2002, 858).
However, the conventional ligand including a heteroatom is still required to have consistently continuous activity on a multimerization reaction and high selectivity during preparing 1-octene or 1-hexene.
Meanwhile, in the conventional ethylene oligomerization, solvents are excessively used, high catalyst activity is exhibited, and high selectivity may be accomplished, however equipments are expanded, and costs and efforts for separating products from solvents are consumed. Therefore, developments on a technique for resolving limitations accompanied with the excessive amount of solvents used are required.